The present invention relates to measurement, filtration or separation of light, and, more particularly, to wavelength measurement or separation of light according to its spectral components.
Compact and cost-effective wavelength meters are in great demand for numerous applications, including instrumentation and wavelength division multiplexed (WDM) communications systems.
An optical wavelength meter is an electronic instrument that measures the wavelength of a light signal input thereto. An optical multi-wavelength meter is one that can simultaneously measure the wavelengths of multiple signals of light such as channels in a WDM system.
A spectrometer is a device which receives a light signal as an input and produces as an output a light signal which is spread out, or dispersed, in space according to the different spectral components, or colors, of the input light signal. A detector attached to the spectrometer can analyze the output signal in order to quantify the amount of each wavelength component present in the input signal. High resolution spectrometers are used in a wide variety of optical applications such as the above motioned WDM systems. High resolution spectrometers are also used as polarization mode dispersion (PMD) sensors, or as measuring devices to obtain the optical power spectrum of any optical field propagating through an optical network.
In life science, wavelength meters and spectrometers are often used as components in optical sensors for the purpose of measuring or monitoring analytes in a sample or in vivo. For example, a fluid sample of unknown analyte content is tested by inserting the sample into a sample chamber where it contacts an analytical element. Using an optical sensor, changes in the optical properties of the analytical element are recorded and used for determining the characteristics of the analyte of interest in the unknown.
In the field of imaging, measuring the spectral content of the illumination source is necessary for the purpose of processing the image to achieve a better resolution or realistic colors. In black and white imaging, for example, the measurement can be performed with a “light meter.” The meter is pointed at the light source, which would be straight up for daylight or towards a spotlight if it were focused on the object of interest. In color imaging and photography, a more sophisticated measurement is to be used. Rather than measuring a single quantity, a wavelength meter or a spectrometer has to measure numerous points across the visual light spectrum and make a graph of the power at each wavelength that it has found. Once this graph is known, then an accurate representation of the original image can be constructed by removing the influence of the light source from the original scene.
Spectrometers and wavelength meters are also used in the field of colorimetry where a precise determination of color content of a sample material is vital to the successful outcome of a project. For example, in the automotive industry, exact color matching is essential when a portion of a vehicle is being painted so that the repainted portion matches the original color of the rest of the vehicle. It is recognized that the ability to repaint only the repaired portion of the vehicle rather than repainting the entire vehicle leads to considerable savings of money, materials and time. In the aerospace industry, one is often interested in an optimum color scheme for an aircraft which minimizes its ability to be detected by the enemy. It is therefore vital that the exact color specified can be provided by the paint supplier.
In general, spectrometers are classified as either scanning or non-scanning spectrometers. While scanning spectrometers can exhibit very high spectral resolutions, the fact that they must be scanned limits their usefulness to the domain of optical signals whose spectrum does not change appreciably over the required scan time. Non-scanning spectrometers provide an attractive method for estimating the power spectral density of an incident optical signal because the far-field pattern, which serves as the spectral estimate, is available more or less instantaneously and therefore can be used to estimate the power spectrum of very short-lived signals.
Spectrometers and wavelength meters often utilize diffraction gratings to diffract incident light and form a far-field optical pattern from which a power spectral estimate may be obtained. Diffraction gratings typically include a plurality of periodically and equally spaced grooves that act as scattering sites to scatter the incident light. The light scattered from each of the grooves creates an interference pattern a distance away from the grating, known as the far-field diffraction or Fraunhofer pattern. The resulting far-field pattern exhibits regions or peaks of high intensity for a given wavelength, which are commonly called diffraction orders.
A typical spectrometer consists of a slit, a collimator lens, a dispersive optic (e.g., diffraction grating), an objective lens or lenses for focusing the various wavelengths and a photometer for measuring the intensity of the various wavelengths. A light source is typically sampled by an optical fiber so that a portion of the light is directed to a slit which expands the light to a beam. The beam is collimated by a lens and impinges on the diffraction grating from which the beam is dispersed at several angles depending on the wavelengths of the light. The wavelengths of interest are then refocused by an objective lens and measured by a photometer.
Wavelength or wavelengths of light may be measured using a device called Michelson interferometer, which measures an interference signal between two light beams generated from the optical signal being measured. More specifically, in such devices a wavefront of light is separated into two beams so as to introduce of an optical path difference between such beams. This causes modulation of light intensity due to interference between the two beams, such that each optical wavelength present in the input light generates its own modulation frequency. Thus, the spectral content of the input light can be decoded by using Fourier transform.
The optical path difference may be introduced, for example, by allowing one light beam to traverse a fixed path, while varying the path of another light beam using a mirror, so that the interference signal is a function of the mirror position. In another method, optical path difference is introduced between two rays with orthogonal polarization directions inside the double-refractive crystal. The waves corresponding to ordinary and extraordinary polarizations separate upon incidence on the crystal and travel with different velocities. After passing through the crystal, the rays exhibit a phase delay between them, which is proportional to crystal thickness. The two rays then interfere with each other after passing through a polarizing analyzer. The resulting intensity variations, which bear the signature of presented spectral components, are transformed or converted into an electrical signal by a photodetector. The electrical signal is thereby recorded for analysis.
Prior art spectrometers and wavelength meters suffer from limited resolution which is inherently imposed by the grating parameter or the refractive crystal characteristics. In addition prior art spectrometers and wavelength meters have rather poor performances in cases where the beam of light is not fully collimated prior to the measurement.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method and apparatus for wavelength measurement or separation of light according to its spectral components devoid of the above limitations.